Awaiting with null coalescing operator thrown exception

Question

I'm encountering some strange behavior.

When running this piece of code:

var foo = await actionContext.RequestContext?.Principal?.ToUserTokenAsync() ?? UserToken.UnidentifiedUser;

Principal is null at runtime and I get a null reference exception.

Why it's not just returning --> UserToken.UnidentifiedUser?

Answer 1

I believe this is a precedence problem. Your code is effectively:

var foo = (await actionContext.RequestContext?.Principal?.ToUserTokenAsync()) ??
          UserToken.UnidentifiedUser;

In other words, if the result of awaiting is null, then return UserToken.UnidentifiedUser. But you still try to do the awaiting - which means you might await something that's null, which fails.

What I suspect you want is:

var task = actionContext.RequestContext?.Principal?.ToUserTokenAsync()
    ?? Task.FromResult(UserToken.UnidentifiedUser);
var foo = await task;

Or avoid awaiting entirely when there's a null:

var task = actionContext.RequestContext?.Principal?.ToUserTokenAsync();
var foo = task != null ? await task : UserToken.UnidentifiedUser;